Matrix emitter for thermionic conversion systems



G. A. HAAS Feb. 11, 1964 MATRIX EMI'ITTER FOR THERMIONIC CONVERSION SYSTEMS 2 Sheets-Sheet 1 Filed June 29., 1962 LEE TEMPERATURE (K) INVENTOR GEOR G E A. H AAS ATTORNEY Feb. 11, 1964 G. A. HAAS 3,121,043

MATRIX EMITTER FOR THERMIONIC CONVERSION SYSTEMS Filed June 29, 1962 2 SheetsSheet 2 uc FURTHER ACTIVATED WITH B0.

4 -l.66+6.l IO T uc ACTIVATED WITH u =2.94+|.|-|0 T l l I l I 5 .6 .e .9 L0 x10" INVENTOR GEORGE A. HAAS BY.MM/

ATTORNEY United States Patent 3,121,048 MATRIX EMHTTER FQR Tl-EIGNIC QUNVERSIGN SYSTEMS George A. Haas, Alexandria, Va., assignor to the United States of America as represented by the Secretary of the Navy Filed June 29, 1962, Ser. No. 207,164

Claims. (Cl. 252-301.i)

(Granted under Title 35, US. Code (1952),sec. 266) The invention .described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to a thermionic emitter and more particularly to a matrix type thermionic emitter which can be fission-heated in a neutron flux to provide high electron emissions.

Recent-progress in the field of thermionic energy conersion has focused attention on-the possibility that elec tric power can be extracted by thermionic emission of electrons. A fissionable element has been proposed to serve as the source of heat and to be arranged in heat transfer relationship with a thermionic emissive material to cause sustained emissions of electrons without the use of an electric filament current. Although previously proposed materials and structures have served to illustrate the general requirements for fission-type emitters, they have not demonstrated the desirable thermionic properties that are expected in the field of thermionic energy conversion.

The present invention provides a matrix type emitter which is both a fissionable material and a copious electron source, and said emitter may therefore be heated in a neutron flux to cause emissions of electrons from its surface for a sustained period ofoperation. The thermionic emitter rnay be utilized in an energy conversion system to achieve an electric power-producing reactor that directly converts fission heat to electrical energy. The matrix emitter of the present invention has been found to be a highly emcient thermionic source, as shown by substantially higher emissions and also by its ability to operate at lower temperau-res than previous emitters.

Accordingly, it is an object of the present invention to provide a thermionically emissive structure having a matrix composition with superior emissive properties than previous emitters of this type.

Another object of this invention is to provide a new electron emitter that can operate effectively at considerably lower temperatures.

A further object of the invention is to provide an improved fissionable matrix composition for use in low field applications, such as the thermionic energy converters.

A still further object of the invention resides in the provision of a matrix emitter in which there is a continuous renewal of the active emissive surface during operation.

Other objects and features of the invention will become apparent to those skilled in the art as the disclosure is made in the following detailed description of a preferred embodiment of the invention as illustrated in the accompanying drawing, in which:

FIG. 1 is a cross section view, in perspective, of a thermionic emitter in accordance with the invention, and

FIGS. 2 and 3 are graphic representations indicating the performance characteristics of the present invention.

Referring now to FIG. 1 of [the drawing, reference numeral 20 is used to designate generally the matrix cath ode structure comprising the present invention. A thin metal cup 21, formed of tantalum or other similar metal, contains therein a matrix composition 22 which is formed from a powdered mixture of ingredients that are pressed together into a binding form and then fused to a solid by means of fission heat caused by a neutron flux. The

matrix composition consists essentially of a fissionable uranium compound, namely, of uranium carbide, UC, and a barium-dispensing compound, such as, barium oxide or barium silicate, but preferably barium calcium aluminate which has been found to be more desirable as a barium dispenser being capable of prolonging as well as stabilizing the performance of the present emitter. Uranium carbide and barium calcium aluminate are mixed together in a fine powdered form in a weight ratio of about 9 to 1, respectively. The matrix mixture also includes a small amount of reducing agent which is present therein in about 0.1 to 1 percent of the total weight. A slight excess of carbon atoms in the UC composition is sufficient to provide a measure of reducing action for the decomposition of the barium compound within the matrix. Reduction may also be achieved by means of other reducing agents which may be included Within the matrix composition 22 or in the cup structure 241, such as, tantalum, tungsten, molybdenum or zirconium hydride.

The surface coating 23 consists of a monolayer of barium, shown in the drawing in an exaggerated thickness for purposes of illustration. A collector plate or anode 24 is held in spaced relationship with respect to the matrix, while a guard ring 25 confines the electron paths from the emitter to the area of the plate. The envelope 26 confines the assembled parts into a highly evacuated space. Three lead-in conductors provide externally accessible terminals: lead 27 is attached to the emitter, lead 2% to the anode and lead 29 to the guard ring 25 to provide a uniform field.

The matrix composition is for-med by combining together the dry, powdered form of uranium carbide and barium calcium aluminate, which have been ground into fine powders and passed through a 400 mesh U.S. sieve. Since uranium carbide is pyrophoric as well as radioactive, the process must be performed in a dry, inert atmosphere. The powder mixture is introduced into the metal cup and pressed therein with a pressure of about 70 tons per square inch. The cup and pressed matrix mixture may then be inserted itno an evacuated envelope for a power-producing reactor or converter device of the type alluded to earlier in the specification.

The good thermionic properties of the present emitter can be inferred from the oriented dipole which is believed to be present on the carbide surface. The emission properties of the carbide are enhanced by free uranium which forms on the surface; they are further enhanced by an adsorbed monolayer of barium. A free barium monolayer is formed on uranium in accordance with the invention by fission heating the matrix to decompose small amounts of aluminate. Barium vapor is then transplanted through the pores of the carbide mixture and migrates out to the surface. The barium metal spreads and covers the surface with an adsorbed monolayer, which provides a dipole arrangement of a more electropositive metal over a less electropositive one. The external surface now constitutes an oriented dipole which substantially lowers the work function of the carbide. During the operation-of the matrix cathode, any barium that may be lost from the surface by evaporation is continuously replenished by diffusion of additional metal from within the matrix.

By employing the above method for fabricating the matrix, the composition will reach sintering temperatures during fission heating at 1200 K. or more, and said composition Will fuse into a solid structure. The reserve barium metal will remain dispersed Within a finely porous matrix.

An initial activation period has been found necessary in which the emission density of the matrix emitter increases and finally becomes constant as the temperature is increased up to about 1400 K. The emission current rises and stabilizes in about an hour. The improved emitter may operate effectively at a temperature in the range of about 800 to 1550 K. to provide substantially higher electron emissions. The matrix emitter is operable in a range of temperatures which heretofore was not possible with other carbide emitters, moreover, fission-type emitters have not been anticipated for operation in the present range of temperatures.

The improvements achieved by means of the present in vention are now indicated graphically with reference to FIG. 2, the current output of the emitter is compared with a solid uranium carbide structure having no active surface and one which retains essentially a uranium monolayer surface. The improvements achieved by the matrix emitter are in the order of 3 in magnitude, which is at least a thousand fold increase in emissions over other similar emitters that were considered for energy converters.

FIG. 3 shows the characteristic Richardson plots for a UC matrix emitter with a uranium surface alone and also one containing an adsorbed monolayer of barium. From the slop and intercept of said plots, values for 4;, representing the efiective work function, or the amount of energy necessary for removing an electron from the surface, can be determined. For the uranium surface alone, at 1250 K. about 3.08 ev. are required to remove an electron, while the barium-activated surface requires about 2.42 ev.

The inclusion of a barium-dispensing compound, such as the aluminate or silicate, is not necessarily restricted to the weight ratio as indicated previously, but it may be used in a relatively wide range of concentrations, an upper limitation being imposed by the fact that its presence forms a contaminant material to the fission properties of uranium carbide. The aluminate may be used in amounts as high as 20% by weight without undue efiects. The calcium in the aluminate has been found to lower the evaporation rate of the barium and thus aids in prolonging the operable life of the matrix. A suitable composition at aluminate may be formed by mixing 0.3 mole CaCO;;, 5 moles BaCO and 2 moles of A1 0 and firing said mixture in air to fusion.

By applying the teachings of the present invention, a fissionable type carbide emitter will provide substantially higher electron emissions. A matrix emitter constructed in accordance with the invention affords long operable life at lower temperatures than those previously considered feasible for carbides.

The foregoing is considered illustrative only of the principle of the invention. Since numerous modifications and changes will occur to those skilled in the art, it is not desired to limit the invention to the exact construction and composition shown and described, and accordingly all suitable modifications and equivalents may be resorted to, falling within the scope of the appended claims.

What is claimed is:

1. A matrix emitter for thermionic conversion systems comprising a pressed powder mixture of a fissionable matrix which becomes heated in a neutron flux, and a barium compound from the group of compounds consisting of barium oxide, barium silicate and barium calcium alumina-te in a weight ratio of 9 to 1, respectively, said mixture including a reducing agent in an amount from about 0.1 to 1 percent by Weight of said mixture, said reducing agent being capable of decomposing said barium compound during the operation of said emitter.

2. A matrix emitter for thermionic conversion systems comprising a pressed powder mixture of uranium carbide and a barium compound from the group of compounds consisting of barium oxide, barium silicate and barium calcium aluminate in a weight ratio of 9 to 1, respectively, said mixture containing a reducing agent in an amount from about 0.1 to 1 percent by weight of said mixture, said reducing agent being capable of decomposing said barium compound during the operation of said emitter.

3. A matrix emitter for thermionic conversion systems comprising a pressed powder mixture of uranium carbide and bmium calcium aluminate in a weight ratio of about 9 to 1, respectively, said mixture being pressed within a container formed of tantalum metal.

4. A matrix emitter for thermionic conversion systems comprising a fused matrix of a fissionable carbide and barium calcium aluminate in a weight ratio of about 9 to 1, respectively, said matrix including therein a small amount of carbon of from about 0.1 to 1 percent by weight of said mixture to provide reducing action to said al uminate during the operation of said emitter.

5. A matrix emitter for thermionic conversion systems comprising a fused matrix of uranium carbide and barium calcium aluminate in a weight ratio of about 9 to 1, re spectively, said matrix including therein a small amount of reducing agent from about 0.1 to 1 percent by weight of said mixture, said reducing agent being capable of decomposing said aluminate during the operation of said emitter.

6. A matrix emitter in accordance with claim 5 in which the reducing agent is zirconium hydride.

7. A matrix emitter in accordance with claim 5 in Which the reducing agent is molybdenum.

8. A matrix emitter in accordance with claim 5 in which said matrix has an electron-emitting surface which consists of a monolayer of barium.

9. A matrix emitter in accordance with claim 5 in which said matrix forms an electron-emitting surface which consists of an absorbed monolayer of barium on a uranium surface.

10. The method of forming a matrix emitter comprising the steps of pressing a powder mixture of uranium carbide, a reducing agent and barium calcium aluminate to form a compact body, placing said body under a vacuum and in the path of a neutron flux to heat said body sufficiently to fuse said mixture and to form a monolayer metal surface thereon.

Nucleonics, vol. 17, No. 7, July 1959, pp. 49-53. Nucleonics, vol. 19, No. 12, December 1961, pp. 66, 70, 72 and 73. 

1. A MATRIX EMITTER FOR TERMIONIC CONVERSION SYSTEMS COMPRISING A PRESSED POWDER MIXTURE OF A FISSIONABLE MATRIX WHICH BECOMES HEATED IN A NETURON FLUX, AND A BARIUM COMPOUND FROM THE GROUP OF COMPOUNDS CONSISTING OF BARIUM OXIDE, BARIUM SILICATE AND BARIUM CALCIUM ALUMINATE IN A WEIGH TRATIO OF 9 TO 1, RESPECTIVELY, SAID MIXTURE INCLUDING A REDUCING AGENT IN AN AMOUNT FROM ABOUT 0.1 TO 1 PERCENT BY WEIGHT OF SAID MIXTURE, SAID REDUCING AGENT BEING CAPABLE OF DECOMPOSING SAID BARIUM COMPOUND DURING THE OPERATION OF SAID EMITTER. 